1. Field of the Invention
The present invention relates to a hydraulic control apparatus for controlling a hydraulically operated continuously variable transmission of belt-and-pulley type used for a motor vehicle.
2. Discussion of the Prior Art
A continuously variable transmission whose speed ratio is continuously variable is known as a transmission incorporated in a power transmitting system for a motor vehicle. An example of a belt-and-pulley type continuously variable transmission is disclosed in laid-open Publication No. 52-98861 of unexamined Japanese Patent Application. This continuously variable transmission includes a first and a second shaft, a pair of variable-diameter pulleys provided on the first and second shafts, respectively, a belt connecting the pulleys, and a pair of hydraulic actuators for changing effective diameters of the pulleys.
For controlling the speed ratio of such a belt-and-pulley type continuously variable transmission, there is provided a hydraulic control apparatus as disclosed in laid-open Publication Nos. 59-159456 and 60-95262 of unexamined Japanese Patent Applications, which incorporates a shift control valve assembly including a directional control valve and a flow control valve each of which uses a spool. The directional control valve is adapted to change the direction in which the speed ratio of the continuously variable transmission is changed, namely, to selectively effect the shift-up and shift-down operations of the transmission. The flow control valve functions to control the speed or rate at which the speed ratio of the transmission is changed in the shift-up or shift-down direction. These directional and flow control valves are operated by an actuating hydraulic pressure which is applied to the spools through respective solenoid-operated valves that are controlled by drive signals generated by a suitable control device.
In controlling the continuously variable transmission, it is generally desired that the continuously variable transmission be slowly shifted up in the event of a trouble associated with the shift control valve assembly, in order to avoid an excessive rise in the speed of the engine. That is, if the transmission is shifted down during a high speed running of the vehicle, the engine speed is forcibly raised. To avoid this phenomenon, the electric control device of the hydraulic control apparatus is usually adapted such that the speed ratio of the continuously variable transmission is relatively slowly changed in the shift-up direction when the two solenoid-operated valves for the directional and flow control valves are both placed in the OFF or de-energized state. In this fail-safe arrangement, the transmission is slowly shifted up in the event of a trouble with the solenoid-operated valves. Conventionally, the spools of the directional and flow control valves are biased by respective springs toward the shift-down position and the flow-restricting position, respectively, whereby the transmission is slowly shifted down in the event that the actuating pressure for operating the spools cannot be applied to the spools for some reason or other. This shift-down of the continuously variable transmission is not desired for the reason stated above.
In the above fail-safe arrangement, whenever a trouble with the solenoid coil of the directional or flow control valve occurs, the speed ratio of the transmission (speed of the input shaft/speed of the output shaft) is lowered toward the smallest value due to the slow shift-up operation. If the vehicle is stopped in this condition, however, the vehicle cannot be smoothly started since a sufficient drive force for starting the vehicle cannot be obtained with the smallest speed ratio of the transmission.
In a hydraulic control apparatus as disclosed in laid-open Publication No. 64-49751, the four combinations of the ON-OFF states of the two solenoid-operated valves for the directional and flow control valves provide four shifting modes of the continuously variable transmission, that is, a slow shift-up mode, a rapid shift-up mode, a slow shift-down mode and a rapid shift-down mode. Further, a medium shift-up mode and a medium shift-down mode are established by operating the solenoid-operated valve for the flow control valve at the controlled duty cycle. The directional control valve has an output port from which a first line pressure is produced when the valve is placed in one of the shift-up modes. This output port is connected to the driving side hydraulic cylinder (one of the two hydraulic actuators), through a by-pass line equipped with a check valve having a flow restricting function. The first line pressure is applied to the driving side hydraulic cylinder primarily through the by-pass line when the directional control valve is placed in the slow shift-up mode. The provision of the by-pass line eliminates a port of the flow control valve for the slow shift-up operation of the transmission, thereby enabling the spool of the flow control valve to have only three lands. This arrangement results in shortening the overall length of the flow control valve, reducing the mass of this valve, and thereby improving the operating response of the valve over a relatively wide range of the flow rate during a duty cycling operation of the corresponding solenoid-operated valve.
However, the known hydraulic control apparatus as disclosed in the above publication No. 64-49751 suffers from relatively low durability of the hydraulic circuit, due to early deterioration of the check valve provided in the by-pass line indicated above. The check valve has a valve member in the form of a ball, and a valve seat on which the ball is seated. The ball and the valve seat are inevitably worn out in a relatively short time, due to repeated abutting contact of the ball with the seat when the slow shift-up and shift-down operations of the transmission are alternately effected.